Control of HIF-Iα expression by elF2α phosphrlation-mediated translational repression

Hypoxia inducible factor la (HIF-Ia) plays a central role in regulating tumor angiogenesis via its effects on vascular endothelial growth factor (VEGF) transcription, and its expression is regulated through proteasome-mediated degra-dation. Paradoxically, previous studies have shown that proteasome inhibitors (PI) block tumor angiogensis by reducing VEGF expression, but the mechanisms have not been identified. Here, we report that PIs down-regulated HIF-Ia protein levels and blocked HIF-Ia transcriptional activity in human prostate cancer cells. PIs induced phosphorylation of the translation initiation factor 2a (eIF2a), which caused general translational repression to inhibit HIF-Ia expression. Furthermore, PIs induced HIF-Ia accumulation in LNCaP-Fro5 cells depleted of eIF2a via siBNA transfection and in MEFs expressing a phosphorylation-deficient mutant form of eIF2a. Finally, PIs failed to induce eIF2a phosphorylation or translational attenuation in DU145 or 2S3JB-V cells, and, in these cells, PIs promoted HIF-Ia accumulation. Our data established that PIs down-regulated HIF-Ia expression in cells that display activation of the unfolded protein response by stimulating phosphorylation of eIF2a and inhibiting HIF-Ia translation.